1. Field of the Invention
This invention relates to photoresists, and more particularly to photoresists that have a low optical density, a high sensitivity towards cross-linking and a controllable dissolution rate in alkaline developers, and most particularly, to photoresist compositions comprising a light sensitive compound in an aqueous alkali soluble binder comprised of an unsaturated dicarboxylic acid anhydride esterified resin such as a phenolic resin, e.g., a novolak type cresolformaldehyde resin or a poly (p-hydroxystyrene).
2. The Prior Art
Photoresist compositions are well known in the art. When exposed to activating radiation, such as ultraviolet photons, the photoresist composition is chemically altered in its solubility to certain solvents (developers). Two types are known. The negative acting resist is initially a composition which is soluble in its developer but following exposure to actinic radiation becomes less soluble in the developer thereby defining a latent image. Positive acting resists work in the opposite fashion, actinic radiation making the resist more soluble in the developer. In resist patterning, photons of shorter wavelengths, e.g. less than about 300 nanometers (nm) are preferably employed because the resolution of printing is limited by diffraction.
Photoresist compositions known to the art generally comprise a light sensitive compound in a film forming polymeric binder. The resin binder most frequently used are the alkali soluble phenolic resins, e.g., cresol-formaldehyde resins. These materials are the product of a reaction of cresol and formaldehyde under conditions whereby a thermoplastic (novolak) polymer is formed. Photoresist compositions are prepared using the novolak type cresol formaldehyde resin by incorporating therein a photoactive compound, a socalled sensitizer, for instance, one of the group of the 4- or 5- substituted naphthoquinone -1,2-diazidesulfonic acid esters.
In the prior art, the above described photoresist compositions using novolak resins as a binder are most often used as masks to protect substrates during the etching step in the manufacture of semiconductors. In such manufacture, the photoresist is coated onto the surface of a semiconductor substrate as a thin film about 0.5 to 3 microns in thickness and then imaged and developed to remove soluble portions thereof. The resist medium remaining on the surface of the substrate is then employed as a protective mask to facilitate the selective etching as by reactive ion etching of the exposed portions of the substrate thereby defining a circuit pattern.
Phenolic resins are advantageously used as the binder in resist formulations because they are resistant to reactive-ion etching processing commonly used in semiconductor manufacturing. However, resists formulated with phenolic resin such as novolac resins or poly(p-hydroxystyrene) are not suitable for exposure with photons of shorter wavelengths (i.e. less than 300 nanometers) because these resins absorb these photons strongly. In order to achieve submicron fine-line lithography, the optical absorption at the exposure region must be low and the photosensitivity high. Novolac resins and poly(p-hydroxystyrene) have low photosensitivity towards deep UV (i.e. 200-300 nanometers) photons. To increase the photosensitivity of these resins photoactive compounds are incorporated in the phenolic-resin based resist formulations. The additional optical absorption of these photoactive compounds disadvantageously effects the low optical absorption requirement for fine line lithography using deep UV exposure. Typically, more than 15 weight percent of photoactive compounds in the resist formulation are required to achieve reasonable resist performance. For example, about 15 to 30 weight percent of napthoquinone -1,2-diazide-sulfonic acid esters are used in novolac type cresol-formaldehyde resins to formulate conventional positive resists, and about 10 to 25 weight percent of aryl bisazides are incorporated in poly(p-hydroxystyrene) to formulate alkali soluble negative resists. The high optical absorption of these photosensitized resists in deep UV region precludes uniform photolysis through the resist films, and consequently high-resolution resist delineation is not achievable. Resins with lower optical absorptivity in deep UV region, such as poly(methacrylic acid-co-methyl methacrylate) and poly(methacrylic acid-co-ethyl methacrylate-costyrene), have been used as the binder material in deep-UV resist formulations, but again the amount of photoactive compound (such as aryl bisazides or naphthoquinone-1,2-diazide-sulfonic acid esters) required to yield reasonable resist performance is too high to meet the low optical-absorption requirement. Furthermore, the etch resistance and thermal stability of resist films based on methacrylic acid-containing resins are inferior to phenolic resins.
Another problem associated with the use of phenolic resins as photoresist materials is that the synthesis of the resins is difficult to control within precise limits so that batch-to-batch variations in resin alkali solubility results. In many cases the photoresist systems exhibit high dissolution rates for development in alkaline solutions normally used as developers. The very high dissolution rates prevent adequate control over processes to obtain fine line configurations. Attempts to decrease the dissolution rate by increasing the concentration of the sensitizer in the photoresist, as has already been discussed increases the optical absorption of the resist to the point where full penetration of the film thickness by actinic radiation may be substantially lost.
The prior art has attempted to use acetylation of the phenolic hydroxyl group of the phenolic resins as a means to reduce the optical absorptivity of the resist formulations based on these resins or to decrease the dissolution rate of the resist films. For example, in copending patent application Ser. No. 560,781 filed Jan. 30, 1984 there is disclosed acetylation of the phenolic hydroxyl groups of novolak resins with an anhydride of an aliphatic monoacid containing 2-5 carbon atoms e.g. acetic anhydride. However, acetylation of the phenolic hydroxyl groups, even at fairly low percentages of conversion results in a partially esterified phenolic resin which is insoluble in alkaline solutions thereby requiring the use of organic solvents as developer solutions. Development of the resist patterns in organic solvents generally causes resist pattern distortion, resulting in poor sub-micron resolution.
There is, therefore, a need in the art to provide alkali-soluble resin binder based photoresists with high etch resistance and low optical absorption in deep ultraviolet (i.e. 200-300 nanometers) and to control the dissolution rate of the photoresist in alkaline developer solutions.